Ferritic stainless steel sheet is excellent in corrosion resistance and heat resistance and is being used for household electrical appliances, transport equipment, building use, and various other fields. However, it is inferior in ductility compared with austenitic stainless steel and suffers from formation of surface relief shapes called “ridging” when worked to shape it. There is therefore the problem of the surface quality and the polishing ability after being worked to shape it being obstructed.
To improve the shapeability, as described in PLT 1, the method is disclosed of reducing the C or N and adding Ti or Nb. By making the steel components high in purity and increasing the {111} crystal orientation, ferritic stainless steel sheet can be improved in the “r-value”, an indicator of deep drawability, and improved in shapeability.
Regarding ridging, it is known that ridging occurs due to colonies of crystal grains which have similar crystal orientations remaining at the finished product sheet due to the casting structure or hot rolling structure. Among these, in particular, numerous arts have been disclosed for reducing colonies which have {100} crystal orientations. As representative arts, there are the electromagnetic stirring, inoculation of solidification nuclei, low temperature casting, etc. which are shown in PLT 2 etc. as techniques for making the solidified structure equiaxial. Further, limits on the hot rolling conditions, annealing conditions, and colony size in finished product sheet are known from PLTs 3 to 5 etc.
In the above way, improving the r-value and reducing the ridging in conventional ferritic stainless steel sheet by adjusting the components and establishing suitable production conditions have been disclosed. In particular, for ridging, a level enabling it to be rendered completely harmless has not yet been reached. It is necessary to control the uneven structure and texture in the direction of sheet thickness and to further improve the surface quality.
On the other hand, PLTs 6,7, and 8 disclose patents relating to Sn-containing ferritic stainless steel. PLT 7 discloses art relating to ferritic stainless steel which is excellent in corrosion resistance and workability and shows, relating to workability, art for giving Sn-containing steel a 0.2% yield strength of 300 MPa or less and elongation at break of 30% or more. However, with just the above 0.2% yield strength or elongation at break, steel which is sufficiently satisfactory in deep drawability and ridging resistance cannot be obtained. Issues remain in workability.